Vello scene only ever grows, so we need to clear it as soon as it's
possible (in clear rect). This PR also adds ignore_clips to
vello_backend (already exists in vello_cpu_backend).
Testing: Behavior is verified by existing tests, as this is mainly a
change for performance. There are currently no performance tests. This
makes bunnymark actually playable (from 3 FPS to 20 FPS on vello_cpu).
---------
Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>
